Constant current control based on indirect current sensing for critical conduction mode buck converter

ABSTRACT

A light fixture includes a converter operable to drive a light source of the light fixture. The buck converter is operable to provide a predetermined average current to the light source or load. A switch of the buck converter is located in the low side of the buck converter such that a controller of the buck converter may drive the switch directly without the use of an isolation transformer. The buck converter controls operation of the switch based on current through the switch and current through the primary inductor of the buck converter.

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the reproduction of the patent document or the patentdisclosure, as it appears in the U.S. Patent and Trademark Office patentfile or records, but otherwise reserves all copyright rights whatsoever.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to and benefit of U.S. ProvisionalPatent Application Ser. No. 61/625,458 filed Apr. 17, 2012, entitled“CONSTANT CURRENT CONTROL BASED ON INDIRECT CURRENT SENSING FOR CRITICALCONDUCTION MODE BUCK CONVERTER” which is incorporated by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates generally to constant current drivercircuits. More particularly, this invention pertains to improved buckconverters for driving light emitting diodes (LEDs) to emit light from alight fixture.

Compared to incandescent lamps and fluorescent lamps, LED lighting has alonger useful life and superior dimming capability. That is, dimming anLED light source will not affect the life span of the LED light source.Thus, as the cost of LED lighting decreases, LED lighting is becomingthe lighting of choice for most applications.

Referring to FIG. 1, an LED light source 104 needs a constant currentdriver circuit to provide a consistent level of light output. A buckconverter 100 is the basis for most constant current LED drivercircuits. The switch 102 (e.g., a MOSFET) in the conventional buckconverter 100 is on the high side floating. A controller 106 of theconverter 100 is typically an average current control integrated circuitthat needs to be referred to the ground of the converter to sense theaverage current through the load (i.e., the LED light source 104). Thus,an isolation transformer is required for the controller 106 to turn theswitch 102 on and off. The primary winding 108 of the isolationtransformer is connected between a gate drive output of the controller106 and a ground of the converter 100. The secondary winding 110 of theisolation transformer is connected between a control terminal of theswitch 102 and a low side of the switch 102. A high side of the switch102 is connected to a positive input of the converter 100. Eliminatingthe isolation transformer would reduce the cost, size, and complexity ofthe converter 100.

BRIEF SUMMARY OF THE INVENTION

Aspects of the present invention provide a buck converter operable toprovide a constant current to a load without an isolation transformerconnecting a gate drive output of a controller of the converter to acontrol terminal of a switch of the converter.

In one aspect, a buck converter is operable to provide a predeterminedaverage current to a load and includes a primary inductor, a primaryinductor current integrator circuit, a switch, and a controller. Theprimary inductor is connected between a positive input of the converterand first load terminal. The primary inductor current integrated circuitsenses current through the primary inductor, provides a primary inductorcurrent signal indicative of the current through the primary inductor,and induces, in the primary inductor, a current equal to an integral ofthe sensed current through the primary inductor. The switch is connectedbetween the load and a ground input of the converter. The controllerreceives the primary inductor current signal and controls the switch asa function of the primary inductor current signal. Controlling theswitch includes turning the switch on and off. The switch conductscurrent through the switch when turned on, and substantially preventscurrent flow through the switch when turned off.

In another aspect, a light fixture is operable to connect to a powersource and provide light. The light fixture includes a housing, a lightsource, and a buck converter. The light source is supported by thehousing and is operable to provide light in response to receivingcurrent. The buck converter is also supported by the housing and isoperable to provide a predetermined average current to the light source.The buck converter includes a primary inductor, a primary inductorcurrent integrator circuit, a switch, and a controller. The primaryinductor is connected between a positive input of the converter and thelight source. The primary inductor current integrated circuit isoperable to sense current through the primary inductor, provide aprimary inductor current signal indicative of the current through theprimary inductor, and induce, in the primary inductor, a current equalto an integral of the sensed current through the primary inductor. Theswitch is connected between the light source and a ground input of theconverter. The controller is operable to receive the primary inductorcurrent signal and control the switch as a function of the primaryinductor current signal. Controlling the switch includes turning theswitch on and off. The switch is operable to conduct current through theswitch when turned on, and the switch is operable to substantiallyprevent current flow through the switch when turned off.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art buck converter connectedbetween the power source and a load.

FIG. 2 is a schematic diagram of a light fixture operable to connect toa power source and provide light including a buck converter operable toprovide a predetermined average current to a light source of the lightfixture.

FIG. 3 is timing diagram of an output voltage of an inverting integratorof the converter, a current through a primary inductor of the converter,and a voltage of the secondary winding of the transformer of the primaryinductor of the converter of FIG. 2.

Reference will now be made in detail to optional embodiments of theinvention, examples of which are illustrated in accompanying drawings.Whenever possible, the same reference numbers are used in the drawingand in the description referring to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

To facilitate the understanding of the embodiments described herein, anumber of terms are defined below. The terms defined herein havemeanings as commonly understood by a person of ordinary skill in theareas relevant to the present invention. Terms such as “a,” “an,” and“the” are not intended to refer to only a singular entity, but ratherinclude the general class of which a specific example may be used forillustration. The terminology herein is used to describe specificembodiments of the invention, but their usage does not delimit theinvention, except as set forth in the claims.

As used herein, “ballast” and “driver circuit” refer to any circuit forproviding power (e.g., current) from a power source to a light source.Additionally, “light source” refers to one or more light emittingdevices such as fluorescent lamps, high intensity discharge lamps,incandescent bulbs, and solid state light-emitting elements such aslight emitting diodes (LEDs), organic light emitting diodes (OLEDs), andplasmaloids.

Referring to FIG. 2, a light fixture includes a housing 300, a lightsource 302, and a buck converter 304. The housing 300 supports the lightsource 302 and the buck converter 304. The light source 302 is operableto provide light in response to receiving current from the converter304. The converter 304 is operable to provide a predetermined averagecurrent to the light source 302.

The converter 304 includes a primary inductor 306, a primary inductorcurrent integrator circuit 308, a switch 310, and a controller 314. Aload (e.g., light source 302) is coupled across first and second loadterminals. The primary inductor 306 is connected between a positiveinput 316 of the converter 304 and the first load terminal. The primaryinductor current integrator circuit 308 is operable to sense currentthrough the primary inductor 306, provide a primary inductor currentsignal indicative of the current through the primary inductor 306, andinduce, in the primary inductor 306, a current equal to an integral ofthe sensed current through the primary inductor 306. The switch 310 isconnected between the light source 302 (via the second load terminal)and a ground input 318 of the converter 304. The controller 314 isoperable to receive the primary inductor current signal and to controlthe switch 310 as a function of the primary inductor current signal.Controlling the switch 310 includes turning the switch 310 on and off.The switch 310 is operable to conduct current when turned on. The switch310 is operable to substantially prevent current flow when turned off.In one embodiment, converter also includes a buck capacitor 330connected in parallel with the load 302 and a buck diode 332 connectedin parallel across the primary inductor 306 and the buck capacitor 330.

In one embodiment, the primary inductor 306 is a primary winding of atransformer. The primary inductor current integrator circuit 308includes a secondary winding 322 of the transformer, an invertingintegrator 370, and a resistive divider network 372. The integrator 370is connected across the secondary winding 322 of the transformer. Anoutput of the integrator 370 is referenced to the ground input 318 ofthe converter 304. The output of the inverting integrator 370 isconnected to a primary inductor current input of the controller 314 viathe resistive divider network 372. The resistive divider network 372matches an output voltage range of the inverting integrator 372 to aninput voltage range of the controller 314. Because the integrator 370 isinverting, the primary inductor current signal received at thecontroller 314 is inversely proportional to the current through theprimary inductor 306. It is contemplated that a non-inverting integratormay also be used if the polarity of the secondary winding 322 isswitched with respect to the polarity of the winding of the primarywinding (i.e., the primary inductor 306) of the transformer.

In one embodiment, the integrator 370 is an op amp integrator includingan op amp 374, an input resistor 376, and a feedback capacitor 378. Thenon-inverting input of the op amp 374 is connected to the secondarywinding 322 via the input resistor 376, and an inverting input of the opamp 374 is connected to the ground input 318 of the converter 304. Theop amp 374 is powered by a voltage regulator (not shown) within theconverter 304. An output of the op amp 374 is connected to thenon-inverting input of the op amp 374 by the feedback capacitor 378.

In one embodiment, the resistive divider network 372 includes a highresistor 380, a low resistor 382, and a filter capacitor 384. The highresistor 380 is connected between the output of the op amp 374 and theprimary inductor current signal input of the controller 314. The lowresistor 382 and the filter capacitor 384 are connected in parallelbetween the primary inductor current signal input of the controller 314and the ground input 318 of the converter 304.

In one embodiment, the converter 304 also includes a switch currentsensing circuit 312 effective to sense an instantaneous current throughthe switch 310 and provide an instantaneous current signal indicative ofthe sensed instantaneous current through the switch 310 to thecontroller 314. The switch current sensing circuit 312 includes asensing resistor 340 connected between the switch 310 and the groundinput 318 of the converter 304. The instantaneous current signal is avoltage of the current sensing resistor 340. In one embodiment, thecontroller determines the peak current through the switch 310 as afunction of the instantaneous current signal provided by the switchcurrent sensing circuit 312. As further explained below, components ofthe converter 304 may be selected such that the peak current is twicethe average current through the load 302, enabling the controller 314 todetermine and control the average current through the load 302.

In another embodiment, the controller 314 turns the switch 310 on whenthe primary inductor current signal indicates zero current through theprimary inductor 306, and turns the switch 310 off a predeterminedperiod of time after turning switch 310 on. The controller 314 thenadjusts the predetermined period of time as a function of the averagecurrent through the light source 302 as determined from theinstantaneous current signal as described above and/or as a function ofthe primary inductor current signal as further described below.

In another embodiment, the controller 314 is operable to determine apeak-to-peak value of the primary inductor current signal. Thecontroller 314 controls the average current through the light source 302as a function of the determined peak to peak value of the primaryinductor current signal. That is, the peak-to-peak value of the primaryinductor current signal is indicative of a peak current through the load302 such that one-half of the determined peak current is the averagecurrent through the load 302. The controller 314 turns the switch 310 onwhen the primary inductor current signal indicates zero current throughthe primary inductor 306 and turns the switch 310 off a predeterminedperiod of time after turning switch 310 on. The controller 314 thenadjusts the predetermined period of time as a function of the averagecurrent through the light source 302 as determined from theinstantaneous current signal and/or the primary inductor current signal.

In another embodiment, the controller 314 turns the switch 310 on whenthe primary inductor current signal indicates zero current through theprimary inductor 306. The controller 314 then turns the switch 310 offwhen the primary inductor current signal indicates that the currentthrough the primary inductor 306 is twice the predetermined averagecurrent.

The controller 314 includes a gate drive output connected directly to acontrol terminal of the switch 310. The controller 314 is operable toprovide a gate drive signal via the gate drive output to the controlterminal of the switch 310 to turn the switch 310 on and off. In oneembodiment, the controller 314 is a critical conduction mode controllersuch as ST Microelectronics' ST L6562. The primary inductor 306 acts asa filter to reduce AC ripple of the current through the load 302, andthe buck capacitor 330 acts as a filter capacitor.

In one embodiment, the converter 304 may also include a referencecurrent circuit 402 operable to provide a reference current to areference current input of the controller 314. In one embodiment, thereference current circuit 402 generates the reference current as afunction of a dimming signal provided to the converter 304. In oneembodiment, the dimming signal is derived by a dimming circuit of thelight fixture as a function of a reduced AC line voltage provided to thelight fixture. That is, the light fixture includes a circuit fordetermining the dimming signal and for providing a reference currentcorresponding to a dimming level indicated by the dimming signal. Thecontroller 314 determines the predetermined average current as afunction of the reference current at the reference current input of thecontroller 314. Thus, a change in the dimming signal (e.g., a change inthe voltage of the AC line voltage) results in a corresponding change inthe average current through the light source 302.

In one embodiment, the light source 302 may be a plurality of lightemitting diodes connected in series with one another. In one embodiment,the housing 300 of the light fixture supports a plurality of converters304 and a plurality of light sources 302, wherein each light source isconnected to one of the plurality of converters. In one embodiment, thelight fixture includes a rectifier 360 connected to the positive input316 and the ground input 318 of the converter 304. The rectifier 360 isoperable to receive alternating current line power and provide a directcurrent power source to the converter 304. It is contemplated that therectifier 360 may provide a direct current power source to one or moreconverters 304 connected to one or more light sources 302.

Referring to FIG. 3, a timing diagram shows the output voltage of theinverting integrator 370 (which is proportional to the output of theresistive divider network 372), the current through the primary inductor306, and a voltage of the secondary winding 322 for the converter 304when operating in the critical conduction mode. The load current is theaverage current through the primary inductor 306. The current throughthe load 302 is expressed in Equation 1.I _(load)=½·I _(peak) ·T·1/T=½·I _(peak)  EQUATION1:

The current through the load 302 is thus one-half of the peak currentthrough the primary inductor 306. The current through the switch 310 isonly part of the current through the primary inductor 306. However,because the peak current through the primary inductor 306 directlycorrelates to the current through the load 302, the current through theswitch 310 can be used to regulate the current through the load 302. Inone embodiment, the reference current is twice the predetermined averagecurrent through the load 302. Thus, the controller 314 turns the switch310 on when the primary inductor current signal indicates zero currentthrough the primary inductor 306 and turns the switch 310 off wheninstantaneous current signal indicates that the current through theswitch 310 is twice the predetermined average current.

Components of the converter 304 should conform with certainspecifications for the converter 304 to operate as described above. Therelationship between the voltage and current of the primary inductor 306is shown in Equation 2.

$\begin{matrix}{{L \cdot \frac{\mathbb{d}i}{\mathbb{d}t}} = V} & {{EQUATION}\mspace{14mu} 2}\end{matrix}$

The current through the primary inductor 306 can be derived byintegration as shown in Equation 3.

$\begin{matrix}{{\int{\frac{V}{L} \cdot {\mathbb{d}t}}} = i} & {{EQUATION}\mspace{14mu} 3}\end{matrix}$

As shown in FIGS. 2 and 3, when the switch 310 is on, the voltage acrossthe primary inductor is V_(in)-V_(out) _(—) _(load) wherein Vin is thevoltage across the positive and ground inputs of the converter 304 andV_(out) _(—) _(load) is the voltage across the load 302. When the switch310 is off, the voltage across the primary inductor 306 is —V_(out) _(—)_(load). Thus, when the switch 310 is on, the current through theprimary inductor 306 is given by Equation 4, and when the switch 310 isoff, the current through the primary inductor is given by Equation 5.

$\begin{matrix}{I_{peak} = {\frac{V_{in} - V_{out\_ load}}{L} \cdot T_{on}}} & {{EQUATION}\mspace{14mu} 4}\end{matrix}$

$\begin{matrix}{I_{peak} = {\frac{V_{out\_ load}}{L} \cdot T_{off}}} & {{EQUATION}\mspace{14mu} 5}\end{matrix}$

If the turns ratio between the primary inductor 306 and the secondarywinding 322 is N, then the voltage across the secondary winding 322 is(V_(in)-V_(out) _(—) _(load))/N when the switch 310 is on, and −V_(out)_(—) _(load)/N when the switch 310 is off.

The relationship between the input voltage and the output voltage of theop amp 374 when the switch 310 is turned off is thus given by Equation6.

$\begin{matrix}{{\int_{0}^{Toff}{\frac{V_{out}}{R\;{2 \cdot C}\;{4 \cdot N}}\ {\mathbb{d}t}}} = I_{peak\_ op}} & {{EQUATION}\mspace{14mu} 6}\end{matrix}$

Simplifying Equation 6 gives Equation 7.

$\begin{matrix}{I_{peak\_ op} = {{\frac{V_{out}}{R\;{2 \cdot C}\;{4 \cdot N}} \cdot T_{off}} = V_{{\_ op}{\_ peak}}}} & {{EQUATION}\mspace{14mu} 7}\end{matrix}$

The relationship between the current and voltage of the op amp 374 isgiven by Equation 8 when the switch 310 is turned on.

$\begin{matrix}{I_{peak\_ op} = {{\frac{V_{in} - V_{out}}{R\;{2 \cdot C}\;{4 \cdot N}} \cdot T_{on}} = V_{{\_ op}{\_ peak}}}} & {{EQUATION}\mspace{14mu} 8}\end{matrix}$

Comparing Equation 5 to Equation 7 (switch 310 off), and Equation 4 toEquation 8 (switch 310 on), it can be seen that when the inductance ofthe primary inductor 306 is equal to the product of the resistance ofthe input resistor 376 and the capacitance of the feedback capacitor378, then the current through the primary inductor 306 can be maintainedin a linear fashion by the primary inductor current integrator circuit308. As shown in FIG. 3, the relationship between the average voltageand peak voltage of the output voltage of the op amp 374 is thus givenby Equation 9.

$\begin{matrix}{V_{avg} = {\frac{1}{2} \cdot V_{{\_ op}{\_ peak}}}} & {{EQUATION}\mspace{14mu} 9}\end{matrix}$

Because the average current going through the load 302 (i.e., the lightsource) is the same average current going through the primary inductor306, the average output voltage of the op amp 374 (and thus the averageoutput voltage of the primary current integrator circuit 308) can beused to sensing control the average current through the load 302.

It will be understood by those of skill in the art that information andsignals may be represented using any of a variety of differenttechnologies and techniques (e.g., data, instructions, commands,information, signals, bits, symbols, and chips may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof). Likewise, thevarious illustrative logical blocks, modules, circuits, and algorithmsteps described herein may be implemented as electronic hardware,computer software, or combinations of both, depending on the applicationand functionality. Moreover, the various logical blocks, modules, andcircuits described herein may be implemented or performed with a generalpurpose processor (e.g., microprocessor, conventional processor,controller, microcontroller, state machine or combination of computingdevices), a digital signal processor (“DSP”), an application specificintegrated circuit (“ASIC”), a field programmable gate array (“FPGA”) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. Similarly, steps of a method orprocess described herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Althoughembodiments of the present invention have been described in detail, itwill be understood by those skilled in the art that variousmodifications can be made therein without departing from the spirit andscope of the invention as set forth in the appended claims.

A controller, processor, computing device, client computing device orcomputer, such as described herein, includes at least one or moreprocessors or processing units and a system memory. The controller mayalso include at least some form of computer readable media. By way ofexample and not limitation, computer readable media may include computerstorage media and communication media. Computer readable storage mediamay include volatile and nonvolatile, removable and non-removable mediaimplemented in any method or technology that enables storage ofinformation, such as computer readable instructions, data structures,program modules, or other data. Communication media may embody computerreadable instructions, data structures, program modules, or other datain a modulated data signal such as a carrier wave or other transportmechanism and include any information delivery media. Those skilled inthe art should be familiar with the modulated data signal, which has oneor more of its characteristics set or changed in such a manner as toencode information in the signal. Combinations of any of the above arealso included within the scope of computer readable media.

This written description uses examples to disclose the invention andalso to enable any person skilled in the art to practice the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to thoseskilled in the art. Such other examples are intended to be within thescope of the claims if they have structural elements that do not differfrom the literal language of the claims, or if they include equivalentstructural elements with insubstantial differences from the literallanguages of the claims.

It will be understood that the particular embodiments described hereinare shown by way of illustration and not as limitations of theinvention. The principal features of this invention may be employed invarious embodiments without departing from the scope of the invention.Those of ordinary skill in the art will recognize numerous equivalentsto the specific procedures described herein. Such equivalents areconsidered to be within the scope of this invention and are covered bythe claims.

All of the compositions and/or methods disclosed and claimed herein maybe made and/or executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of the embodiments included herein, it willbe apparent to those of ordinary skill in the art that variations may beapplied to the compositions and/or methods and in the steps or in thesequence of steps of the method described herein without departing fromthe concept, spirit, and scope of the invention. All such similarsubstitutes and modifications apparent to those skilled in the art aredeemed to be within the spirit, scope, and concept of the invention asdefined by the appended claims.

Thus, although there have been described particular embodiments of thepresent invention of a new and useful A LOW COST CONSTANT CURRENT DRIVERBASED ON CRITICAL CONDITION MODE BUCK CONVERTER it is not intended thatsuch references be construed as limitations upon the scope of thisinvention except as set forth in the following claims.

What is claimed is:
 1. A buck converter operable to provide apredetermined average current to a load, said buck converter comprising:first and second load terminals; a primary inductor connected between apositive input of the converter and the first load terminal; a primaryinductor current integrator circuit functional to sense current throughthe primary inductor, provide a primary inductor current signalindicative of the current through the primary inductor, induce, in theprimary inductor, an integral of the sensed current through the primaryinductor; a switch connected between the second load terminal and aground input of the converter; and a controller operable to receive theprimary inductor current signal and to control the switch as a functionof the primary inductor current signal, wherein controlling the switchcomprises turning the switch on and off, the switch is operable toconduct current through the switch when turned on, and the switch isoperable to substantially prevent current flow through the switch whenturned off.
 2. The converter of claim 1, further comprising: a switchcurrent sensing circuit effective to sense an instantaneous currentthrough the switch and provide an instantaneous current signalindicative of the sensed instantaneous current through the switch to thecontroller; the switch current sensing circuit comprises a sensingresistor connected between the switch and the ground input of theconverter and the instantaneous current signal is a voltage of thecurrent sensing resistor; and the controller is further operable toreceive the provided instantaneous current signal and control the switchas a function of the primary inductor current signal and theinstantaneous current signal, wherein controlling the switch comprisesturning the switch on when the primary inductor current signal indicateszero current through the primary inductor, and turning the switch offwhen the instantaneous current signal indicates that the current throughthe switch is twice the predetermined average current.
 3. The converterof claim 1 wherein the controller is further operable to determine apeak-to-peak value of the primary inductor current signal and controlthe current through the load as a function of the determined peak topeak value of the primary inductor current signal, wherein thecontrolling the switch comprises turning the switch on when the primaryinductor current signal indicates zero current through the primaryinductor, turning the switch off a predetermined period of time afterturning the switch on, and adjusting the predetermined period of time asa function of the determined peak to peak value of the primary inductorcurrent signal.
 4. The converter of claim 1, wherein controlling theswitch comprises: turning the switch on when the primary inductorcurrent signal indicates zero current through the primary inductor; andturning the switch off when the primary inductor current signalindicates a current through the primary inductor that is twice thepredetermined average current.
 5. The converter of claim 1, furthercomprising a reference current circuit operable to provide a referencecurrent to a reference current input of the controller, wherein thecontroller is further operable to determine the predetermined averagecurrent as a function of the reference current.
 6. The converter ofclaim 1 further comprising: a transformer with the primary inductorcomprising a primary winding of the transformer; and the primaryinductor current integrator circuit comprises a secondary winding of thetransformer, an inverting integrator connected across the secondarywinding of the transformer, an output of the inverting integrator isreferenced to the ground input of the converter, and the output of theinverting integrator is connected to a primary inductor current input ofthe controller to provide the primary inductor current signal to thecontroller.
 7. The converter of claim 1, wherein the primary inductor isa primary winding of a transformer, and the primary inductor currentintegrator circuit comprises: a secondary winding of the transformer;and an inverting integrator connected across the secondary winding ofthe transformer, wherein: an output of the integrator is referenced tothe ground input of the converter; the output of the invertingintegrator is connected to a primary inductor current input of thecontroller via a resistive divider network operable to match an outputvoltage range of the inverting integrator to an input voltage range ofthe controller; and the primary inductor current signal is inverselyproportional to the current through the primary inductor.
 8. Theconverter of claim 1, wherein the controller comprises a gate driveoutput connected to a control terminal of the switch, wherein thecontroller is operable to provide a gate drive signal to the controlterminal to turn the switch on and off.
 9. A light fixture operable toconnect to a power source and provide light, said light fixturecomprising: a housing; a light source supported by the housing, saidlight source operable to provide light in response to receiving current;a buck converter operable to provide a predetermined average current tothe light source, said buck converter comprising a primary inductorconnected between a positive input of the converter and the lightsource, a primary inductor current integrator circuit operable to sensecurrent through the primary inductor, provide a primary inductor currentsignal indicative of the current through the primary inductor, andinduce, in the primary inductor, an integral of the sensed currentthrough the primary inductor, a switch connected between the lightsource and a ground input of the converter; and a controller operable toreceive the primary inductor current signal and control the switch as afunction of the primary inductor current signal, wherein controlling theswitch comprises turning the switch on and off, the switch is operableto conduct current through the switch when turned on, and the switch isoperable to substantially prevent current flow through the switch whenturned off.
 10. The light fixture of claim 9, wherein the converterfurther comprises: a switch current sensing circuit effective to sensean instantaneous current through the switch and provide an instantaneouscurrent signal indicative of the sensed instantaneous current throughthe switch to the controller, wherein the switch current sensing circuitcomprises a sensing resistor connected between the switch and the groundinput of the converter and the instantaneous current signal is a voltageof the current sensing resistor; and the controller is further operableto receive the provided instantaneous current signal and control theswitch as a function of the primary inductor current signal and theinstantaneous current signal, wherein controlling the switch comprises:turning the switch on when the primary inductor current signal indicateszero current through the primary inductor; and turning the switch offwhen the instantaneous current signal indicates that the current throughthe switch is twice the predetermined average current.
 11. The lightfixture of claim 9, wherein the controller is further operable todetermine a peak to peak value of the primary inductor current signaland control the average current through the light source as a functionof the determined peak to peak value of the primary inductor currentsignal, wherein controlling the switch comprises: turning the switch onwhen the primary inductor current signal indicates zero current throughthe primary inductor; and turning the switch off a predetermined periodof time after turning the switch on; and adjusting the predeterminedperiod of time as a function of the determined peak to peak value of theprimary inductor current signal.
 12. The light fixture of claim 9,wherein controlling the switch comprises: turning the switch on when theprimary inductor current signal indicates zero current through theprimary inductor; and turning the switch off when the primary inductorcurrent signal indicates a current through the primary inductor that istwice the predetermined average current.
 13. The light fixture of claim9, further comprising a reference current circuit operable to provide areference current to a reference current input of the controller,wherein the controller is further operable to determine thepredetermined average current as a function of the reference current.14. The light fixture of claim 9, wherein the primary inductor is aprimary winding of a transformer, and the primary inductor currentintegrator circuit comprises: a secondary winding of the transformer;and an inverting integrator connected across the secondary winding ofthe transformer, wherein: an output of the integrator is referenced tothe ground input of the converter; and the output of the integrator isconnected to a primary inductor current input of the controller toprovide the primary inductor current signal to the controller.
 15. Thelight fixture of claim 9, wherein the primary inductor is a primarywinding of a transformer, and the primary inductor current integratorcircuit comprises: a secondary winding of the transformer; and aninverting integrator connected across the secondary winding of thetransformer, wherein: an output of the integrator is referenced to theground input of the converter; the output of the inverting integrator isconnected to a primary inductor current input of the controller via aresistive divider network operable to match an output voltage range ofthe inverting integrator to an input voltage range of the controller;and the primary inductor current signal is inversely proportional to thecurrent through the primary inductor.
 16. The light fixture of claim 9,wherein: the controller comprises a gate drive output connected to acontrol terminal of the switch, wherein the controller is operable toprovide a gate drive signal to the control terminal to turn the switchon and off; the light source comprises a plurality of light emittingdiodes connected in series with one another; and the light fixturefurther comprises a rectifier connected to the positive input and theground input of the converter, wherein the rectifier is operable toreceive alternating current line power and provide a direct currentpower source to the converter.
 17. A method of providing a predeterminedaverage current to a load, said method comprising: sensing, via aprimary inductor integrator circuit, a current through a primaryinductor of a buck converter connected to the load, wherein the primaryinductor is connected between a positive input of the converter and theload; providing a primary inductor current signal indicative of thecurrent through the primary inductor from the primary inductorintegrator circuit to a controller of the converter; inducing, via theprimary inductor integrator circuit, an integral of the sensed currentthrough the primary inductor in the primary inductor; receiving theprimary inductor current signal at the controller; and controlling aswitch of the converter via the controller as a function of the primaryinductor current signal, wherein: controlling the switch comprisesturning the switch on and off; the switch is operable to conduct currentthrough the switch when turned on; and the switch is operable tosubstantially prevent current flow through the switch when turned off.18. The method of claim 17, further comprising: sensing an instantaneouscurrent through the switch via a switch current sensing circuit;providing an instantaneous current signal from the switch currentsensing circuit to the controller indicative of the sensed instantaneouscurrent through the switch, wherein the switch current sensing circuitcomprises a sensing resistor connected between the switch and the groundinput of the converter and the instantaneous current signal is a voltageof the current sensing resistor; and receiving the providedinstantaneous current signal at the controller; and controlling, via thecontroller, the switch as a function of the primary inductor currentsignal and the instantaneous current signal, wherein controlling theswitch comprises: turning the switch on when the primary inductorcurrent signal indicates zero current through the primary inductor; andturning the switch off when the instantaneous current signal indicatesthat the current through the switch is twice the predetermined averagecurrent.
 19. The method of claim 17, further comprising: determining,via the controller, a peak to peak value of the primary inductor currentsignal; and controlling, via the controller, the average current throughthe load as a function of the determined peak to peak value of theprimary inductor current signal, wherein controlling the switchcomprises: turning the switch on when the primary inductor currentsignal indicates zero current through the primary inductor; and turningthe switch off a predetermined period of time after turning the switchon; and adjusting the predetermined period of time as a function of thedetermined peak to peak value of the primary inductor current signal.20. The method of claim 17, wherein controlling the switch comprises:turning the switch on when the primary inductor current signal indicateszero current through the primary inductor; and turning the switch offwhen the primary inductor current signal indicates a current through theprimary inductor that is twice the predetermined average current.